1. Field of the Invention
The present invention relates to a pot core transformer and, in particular, a pot core transformer comprised of two halves wherein both of the halves have posts that form the center of the core of the transformer and the posts have holes formed therein that receive an alignment member wherein the alignment member facilitates correct alignment of the two halves of the transformer core.
2. Description of the Related Art
Transformers, and in particular pot core transformers, are commonly used in computer circuits and in communication circuits. These types of devices are used for impedance matching and also for isolation purposes to ensure that data and communication signals within networks are accurately transferred along the network. One such application of these devices is for isolation purposes in ISDN networks.
For these types of applications, it is important that the pot core transformer have inductances that are within very exact tolerances. For example, it is desirable that each pot core transformer have an inductance that is within .+-.5% of the nominal inductance of the device. In these applications, the nominal inductance is typically on the order of 20 to 50 mH. Consequently, great care must be taken during the assembly of the pot core transformer to ensure that the device has an inductance within the desired tolerances.
U.S. Pat. No. 3,609,615 to Parker et al. discloses a typical pot core transformer device. As shown, the pot core transformer device has a casing that forms a core that is comprised of two halves, each of which defines a cylindrical opening with a center post extending in the middle of the cylindrical opening. Further, there is a bobbin assembly upon which the windings of the transformer are wound. The bobbin assembly fits within the cylindrical openings of the two halves with the posts centered inside of the bobbin. Hence, the two halves form the core of the transformer which permits transfer of magnetic flux from the primary winding to the secondary winding on the bobbin assembly.
As shown in U.S. Pat. No. 3,609,615, the two halves of the core are then connected together by positioning a screw or bolt so as to extend through an opening formed in both halves of the core. The bolt preferably extends through the center of the posts on either half of the core and is then tightened against a nut to secure the halves of the core together. An adhesive is positioned on the mating surfaces of the two halves of the core and the two halves of the core remain secured together by the bolt and nut while the adhesive dries. The bolt is apparently tightened while the inductance of the device is simultaneously monitored until a desired inductance reading is obtained. Once the adhesive dries, the nut and bolt are then removed and the device is completed.
The pot core transformer illustrated and described in U.S. Pat. No. 3,609,615 describes the typical pot core transformer that is currently in use. There are some disadvantages of this type of pot core transformer and the above-described assembly method. In particular, the assembly method is expensive as the bolt has to be positioned within the pot core and secured with the nut so that the two halves of the pot core casing can be connected together. This task is often performed by hand which results in a relatively expensive assembly step. There is, of course, additional assembly expense in removing this nut and bolt once the adhesive has dried. Moreover, since it is typically desirable that the outer surfaces of the pot core transformer be relatively smooth, the holes that receive the bolt must then be filled at additional expense. Consequently, these types of pot core transformers have a relatively high assembly cost.
Further, these types of pot core transformers are often difficult to assemble while maintaining the inductance of the device in close tolerance with the nominal or desired inductance. In particular, it is understood that the cross-sectional area of the core, and, in particular, the center of the core comprised of the two posts positioned adjacent to each other, has a significant value on the overall inductance of the device. Consequently, even slight misalignment of the two posts can result in a change in the effective cross-sectional area of the core which can result in variations of the inductance of the device. This misalignment can be the result of the manufacturing tolerances of the openings that receive the bolt or the manufacturing tolerances of the bolt itself. During tightening, a gap between the bolt and the inner walls of the openings can result in slight lateral movement between the two halves of the core. This can, in turn, result in one post being slightly misaligned with the other post thereby altering the effective cross-sectional area of the core and affecting the inductance of the device.
In U.S. Pat. No. 3,609,615, the bolt and nut are tightened while the inductance is simultaneously being measured. Hence, the effect of misalignment of the two posts on the inductance is less of a problem when the inductance is being simultaneously measured. However, simultaneously measuring the inductance while assembling the device is expensive from a manufacturing standpoint and is not generally feasible for devices that are mass produced.
Typically, for mass produced devices, the screw and the nut are tightened to a preset degree which is expected to yield a particular desired inductance. Alignment between the two posts comprising the core is basically performed by the assembler assembling the two halves together by hand in close proximity to each other. Hence, alignment between the two halves of the core is not very precise and is subject to misalignment resulting from the manufacturing tolerances of the device and the skill of the assembler. This results in pot core transformers having actual inductances that may vary widely from the desired inductance of the device.
A further difficulty with pot core transformer devices that are similar to the device disclosed in U.S. Pat. No. 3,609,615, is that the opening that receives the bolt can disrupt the smooth upper surface of the device, even if it is subsequently filled. Typically, when this opening is filled, there still is a divot that remains. In the assembly of many PC boards, vacuum placement devices are used to automate the assembly process. However, the divot that often remains in many prior art pot core transformers can result in the vacuum placement device not being able to securely capture the transformer and place it on a PC board. Further, while there are some other pot core transformers that do not have the alignment holes described above, alignment and assembly of these devices are complicated.
From the foregoing, it should be apparent that there is a need for a pot core transformer device wherein the assembly of the pot core transformer device is simplified. To this end, there is a need for a device which facilitates the alignment between the two halves of the pot core transformer device so that the two halves of the core are more precisely aligned. Lastly, there is a need for a pot core transformer device that can be assembled so as to have close alignment between the posts comprising the core, which still has a planer top surface to facilitate vacuum placement of the device on a printed circuit board.